杏吧原创

NEVER trust a physicist to tell you the time, says Marina Cortes at the University of Lisbon in Portugal. 鈥淧hysics has a slightly different idea about what time is.鈥�

We used to think we had it nailed: time was the tick-tock of a clock somewhere outside the universe against which all processes within it could be measured. This appealing, intuitive idea of an absolute time underpins things like Newton鈥檚 classical laws of motion, and even the distinctly non-intuitive workings of quantum equations, our best description of the nitty-gritty of reality.

Never mind where these external readings of time would come from, Einstein鈥檚 theories of relativity blew away the whole idea. Einstein showed that space and time are, well, relative. Both are part of a unified space-time that is warped by both gravity and motion so that no two observers can ever fully agree on what happened when.

The discrepancies are imperceptible to us because we live our whole lives in roughly the same gravitational field and at roughly the same low speeds. We are deceived into thinking time is absolute by an accident of circumstance, says physicist Carlo Rovelli at Aix-Marseille University in France, author of The Order of Time . 鈥淚n our experience, time passes at the same rate. But this is only true in the non-relativistic approximation in which we live.鈥�

So rule one in thinking clearly about time: cast off the idea that it always ticks at the same rate. Rule two: be prepared to deny it ticks at all.

In our experience, we are carried in the coracle of an eternal present down an inexorable stream of time from past to future. But physics says 鈥渘o鈥� to time鈥檚 flow. Quantum theory indicates that things should work just as well backwards as forwards. Meanwhile, relativity鈥檚 difficulty putting events in any one unambiguous order leads physicists to suggest that reality is a static four-dimensional block of space-time, in which all of time exists all at once.

鈥淩ule 1: Forget time flows at the same rate. Rule 2: Forget it flows at all鈥�

Most physicists explain away the illusion that time flows by appealing to the ineluctable rise of entropy (see 鈥�How to think about鈥� Entropy鈥�). The universe must have started in an implausibly ordered configuration, and what we experience as time is the constant drift away from this state.

So while things may be flowing in the block universe, time isn鈥檛. 鈥淭ime is the direction on that block in which physics tells its most compact, powerful narrative,鈥� says philosopher of science at the University of California, San Diego. Reality as we are experiencing it 鈥渘ow鈥� is like the page of a book. 鈥淵ou turn pages in the direction the laws evolve,鈥� says Callender.

Sorted? Not quite. Conceptions of time in modern physics are at odds not just with our intuitions, but with each other. That鈥檚 hardly any surprise, given that relativity and quantum theory famously fail to agree on anything. A more precise idea of how time works depends on finding a theory of quantum gravity to unify them 鈥� time听a distant dream.

But that might not matter. 鈥淲e can use our approximate notion of time in everyday life and it is fine,鈥� says Rovelli. 鈥淛ust like we can think that Earth is flat in everyday life, and that it doesn鈥檛 rotate.鈥� When it comes to deeper answers, your guess is as good as anyone鈥檚, says Cortes. 鈥淭ime is for me the most mysterious aspect of nature,鈥� she adds. 鈥淚f I knew how to think about it, I鈥檇 retire.鈥�

This article appeared in print under the headline 鈥淗ow to think about鈥� Time鈥�